1. Field of the Invention
This invention relates generally to the design of wastewater and stormwater systems using soil infiltration, and more particularly, to a simplified apparatus and method for determining the maximum soil hydraulic conductivity of soils found above the water table.
2. Related Art
Soil is used to accept wastewater every day from well over 25 million homes in the United States. In addition, soil""s infiltration capacity is used everyday in trenches for storm water runoff and for centralized irrigation systems. Currently, standard percolation tests and/or soils analysis by direct observation and field soil texture determinations is used to determine how to design trenches and other soil based treatment methods for handling wastewater and to estimate the hydraulic capacity of the soil. Significant levels of error occur in such conventional percolation testing procedures including human error, measurement error, and variability in the test procedure due to the falling head pressure as the water drops in a percolation test hole. Soils analysis in the field is a subjective test procedure relying solely on the skill of the individual practitioner in properly characterizing the soil and then xe2x80x9cinferringxe2x80x9d the soil hydraulic capacity from the soils analysis. These older methods fail to take into account such problems as compaction of the soil, and also require highly skilled practitioners to get a reasonably reliable guess of the soil""s ability to accept wastewater for treatment. These sources of testing error result in high failure rates for wastewater methods using soil based treatment and disposal and significant errors in estimating maximum hydraulic capacity of unsaturated soils near the ground surface.
Permeameters have been designed and used to determine soil infiltration capacity, however, the art has not developed simplified design methods for in-the-field applications. One conventional permeameter apparatus that defines the prior art is a permeameter apparatus that was originally devised by Mr. David Pask about 20 years ago. The fundamental principle of this device is based upon Marriottes Apparatus first devised in the 18th Century and results in a constant head water column in the soil hole being tested. However, adoption of this device and earlier methods has not occurred because of difficulty of use of the earlier methods. In addition, similar devices require careful setup and adjustable versions were prone to jamming and leakage. As an example, the Guelph Permeameter used by soil scientists has multiple joints that must be air and watertight and uses a separate air tube to determine the water level in the hole augered in the ground. The Guelph permeameter also has the disadvantage of requiring a perfectly vertical setup.
Other earlier methods include a nomogram developed by David Pask that had flaws in its application which prevented its adoption in the United States. An application of this Pask developed nomogram method includes the application of this different design methodology with an earlier public domain version of the present permeameter. In addition, Amoozegar""s Permeameter described in 1989, has a complicated series of tubes to provide the constant level (head) of water in the hole. Thus, the Amoozegar permeameter uses a more complicated Mariottes apparatus that is not well suited to field applications and has no connection to design methodology. Other examples of apparatus and methods for determining soil hydraulic capacity using constant head methods include Stewart, U.S. Pat. No. 5,322,629, and Ankeny et al, U.S. Pat. No. 4,884,436. These apparatuses are comparable only in the sense that they use constant head arrangements or sometimes require the use of excessive quantities of water for testing (Stewart) but once again provide no methodology. Numerous other patents on methods using permeameters to estimate hydraulic conductivity can be found. However, these methods usually require collection of soil samples and processing in the laboratory, injection of gas into the soil, use of semipermeable membranes to estimate conductivity based on pressure differentials, etc. These other methods result in either: potential errors from disturbing or modifying conditions in the native soil, or the targeting of hydraulic conductivity below the water table or in the bedrock, and both are outside the scope of this invention.
In addition to the above, conventional approaches to onsite system design for treating wastewater and storm water are driven by xe2x80x9cprescriptivexe2x80x9d regulations. That is, the designers of such onsite systems merely apply state regulations and guidelines in their designs. The disadvantages with such an approach is that the specific attributes and characteristics of the target property and the type of soil are not taken into account, often resulting in failure of the system to handle the amount of wastewater.
Therefore, there is a need for a simplistic permeameter apparatus with an accompanying design methodology based on soil performance that can be used easily in the field and is accurate as to estimating the tested soils permeability and maximum hydraulic capability.
The present invention solves the problems encountered with conventional permeameter apparatuses and methods with an improved, simplified permeameter for use in soil infiltration applications such as onsite wastewater and storm water treatment systems. The permeameter of the present invention is a hollow tube having a second tube slidably disposed within an internal chamber of the hollow tube. The second tube also has an air hole and a slot which are used in causing water contained in the hollow tube to seep into an auger hole. A reducing connector is secured to the bottom of the hollow tube which provides the means for maintaining the second tube within the internal chamber of the hollow tube. The second tube is secured in the extended position by a compression fitting attached to the reducing connector. A conventional measuring tape, having consecutive markings showing continuous xc2xc, xc2xd, and 1 inch distances, is affixed to the outside surface of the hollow tube such that the numbers extend vertically along the length, or longitudinal central axis, of the hollow tube.
The permeater of the present invention also uses multiple tables, each of which correlates a specific reading of rate of fall rate with a unique soil hydraulic capacity. The tables are calculated according to conventional soil absorption principals while taking into account the dimensions of the permeameter, the dimensions of the auger hole, and the level of the water line in the auger hole. When used in the field, a user simply determines a stabilized rate of falling water within the hollow tube and uses this stabilized rate to xe2x80x9clook upxe2x80x9d in the table a corresponding soil hydraulic capacity of the tested soil. This methodology allows the user to base his/her analysis and design of onsite systems for treating wastewater and storm water on the performance of the soil, and not merely on prescriptive regulations.
The apparatus and method of this invention provides an efficient and accurate means for determining soil permeability (also called hydraulic conductivity). Results of testing with the method and apparatus of this invention are used to make a number of site design decisions, including bottom area of trench required, amount of water each trench can accept given the site constraints, how far apart the trenches should be once the loading for each trench is known, and whether a soil absorption system is acceptable at that site.
The apparatus of this invention is an improved version of Marriottes Apparatus in which a small diameter hole in the wall of the tube replaces the internal vent tube. This small diameter hole controls the entry of air into the reservoir to release the flow of water according to the demand. In addition, the apparatus includes a chart for instantly converting the fall in reservoir height for saturated soil conditions (measured in minutes per inch) to trench design loading rate (xe2x80x9cDLR Trenchxe2x80x9d). The apparatus may optionally include additional charts converting the raw fall in reservoir height data into other useful information such as, but not limited to, maximum design loading rate for the soil (xe2x80x9cMDLR Soilxe2x80x9d), trench hydraulic capacity (xe2x80x9cTHCxe2x80x9d), or trench separation.
In operation, the permeameter of this invention is used to determine, among other things, the trench design loading rate (xe2x80x9cDLRxe2x80x9d). Once the proper location(s) for testing soil permeability has been established, the permeameter is filled with water, inverted, and inserted into a hole augered into the soil. In a preferred embodiment, a 3 inch diameter hole is bored into the upper horizon of the soil to a depth of 18 to 20 inches using a hand auger. After a period of several minutes, preferably about 5 minutes to about 10 minutes, a stop watch is used to time the fall of the water level in the reservoir. Once the minutes/inch rate of fall is consistent for each reading time period, a steady state of saturated soil conditions around the auger hole has been established. This rate of fall data is then used to determine the previously mentioned site characteristics, i.e., DLR Trench, MDLR Soil, THC, and trench separation.
An advantage of the invention is that saturated conditions are reached quickly around a test hole because of the test holes"" small size.
Another advantage of the invention is that whether saturation conditions have been reached can be easily determined by monitoring the drop of the water level in the permeameter. As a result, soil percolation testing using the permeameter of the present invention can be done much more rapidly than standard percolation testing.
Another advantage of the invention is that the results of the testing with the permeameter of the present invention can be used to make a number of site design decisions, such as, but not limited to bottom area of trench required, how much water each trench can accept given the site constraints, how far apart the trenches should be once the loading for each trench is known, and whether a soil absorption system is acceptable for the site.
Another advantage of the invention is that various useful results can be obtained simply by looking to a supplemental chart instead of requiring sophisticated data interpretation.
Another advantage of the invention is that it is ideal for field use, and it can be easily used and interpreted by a novice operator.
Another advantage of the invention is that the permeameter can be used either in a vertical or tilted arrangement in an auger hole.